A so-called spread spectrum communication (SSC) scheme is a scheme for diffusing the frequency spectrum of a transmission signal using a pseudo noise (PN) code. The SSC scheme only requires a low transmission power density per unit frequency bandwidth and is excellent in privacy, confidentiality, and resistance to interference. In spread spectrum communication, digital data to be transmitted is spread (spread modulation) to a wide frequency band using the PN (Pseudo Noise) code, and the modulated digital data is transmitted from the transmitter. The receiver generates a PN code (to be referred to as an SS reference signal hereinafter) having an image relationship with the PN code on the transmitter. The receiver correlates between the SS reference signal and the receiver input signal, then forms a correlation peak sequence. This correlation peak sequence is detected and waveshaped to demodulate the transmission data. A correlator for calculating the above correlation is a SAW (Surface Acoustic Wave) device such as a SAW convolver or a matched filter.
As this spread spectrum communication apparatus, a transmitter for performing frequency shift keying (FSK) modulation (primary modulation) using a VCO (Voltage-Controlled Oscillator) and then performing direct spread modulation (secondary modulation) using a PN code to transmit data is described in, e.g., Tsubouchi et al., "Asynchronous SSC Transceiver Using SAW Convolver", (Study Meeting of Spread Spectrum Communication of the Institute of Electronics and Communication Engineers of Japan, Apr. 1 and 2, 1988, SS88-7). This reference also describes a receiver comprising a front-end unit for reducing noise from a receiver input signal and amplifying the resultant signal, a correlator for calculating a correlation with a SS reference signal, a detector for detecting the envelope of an output from the correlator, and a waveshaper for waveshaping the detection output.
This SSC apparatus performs transmission by using a frequency obtained by frequency-conversion and frequency-multiplication, as a carrier (center) frequency, which means one component of the FSK signals, e.g., an FSK signal component corresponding to high ("H") level of digital data (base-band information) to be transmitted. On the other hand, the receiver has the SS reference signal to be correlated with the receiver input signal on the basis of an assumption that data to be transmitted is transmitted at this center frequency.
The VCO controls the frequency by mainly using a variable capacitive element (variable capacitor) constituting an inductance-capacitance (LC) oscillator. The frequency undesirably changes with a change in temperature and jitters may be generated in high-speed digital signal transmission because inductance (L) and capacitance (C) are used.
When the carrier frequency changes with a change in VCO oscillation frequency, the correlation output of the receiver decreases substantially. FIG. 6 shows a correlation output level obtained upon reception by a receiver which has an SS reference signal using a center frequency of 200 MHz while the carrier frequency is changed on the transmitter. It is apparent that the correlation output level drops by 10 to 12 dB with a slight change in carrier frequency by 0.05% (0.1 MHz).
The jitters problem generated during high-speed digital signal transmission can be solved by FSK modulation using a direct digital synthesizer (to be referred to as a DDS hereinafter), as proposed in Japanese Patent Laid-Open No. Hei 4-302553 filed by the assignee of the present application. A quartz oscillator can be used as a clock source to drive the DDS to solve the problem posed by the change in frequency.
As described above, using the quartz oscillator and the DDS in the FSK modulation unit, the problems of the change in frequency caused by the change in temperature and of the jitters during high-speed digital signal transmission can be solved. In this case, the frequent occurrence of bit errors during high-speed digital signal transmission is left as another unsolved problem.